1. Field of the Invention
This invention relates to a method of fracturing subterranean formations penetrated by a wellbore utilizing aqueous fluids that may include carbon dioxide or nitrogen or hydrocarbon fluids that may include nitrogen. More particularly, this invention relates to a method of fracturing a subterranean formation and transporting proppant into created fractures utilizing substantially uniform density fluids.
2. Description of the Prior Art
The treatment of subterranean formations penetrated by a wellbore to stimulate the production of hydrocarbons therefrom or the ability of a formation to accept injected fluids has long been known in the art. One of the most common methods of increasing productivity of a hydrocarbon-bearing formation is to subject the formation to a fracturing treatment. This treatment is effected by injecting a liquid, gas or two-phase fluid which generally is referred to as a fracturing fluid down the wellbore at sufficient pressure and flow rate to fracture the subterranean formation. A proppant material such as sand, fine gravel, sintered bauxite, glass beads or the like can be introduced into the fractures to keep them open. The propped fracture provides larger flow channels through which an increased quantity of a hydrocarbon can flow, thereby increasing the productive capability of a well.
A traditional fracturing technique utilizes a water or oil-based fluid to fracture a hydrocarbon-bearing formation.
Another successful fracturing technique has been that known as "foam-fracturing." This process is described in, for example, U.S. Pat. No. 3,980,136. Briefly, that process involves generation of a foam of a desired "Mithcell quality" which then is introduced through a wellbore into a formation which is to be fractured. The composition of the foam is such that the Mitchell foam quality at the bottom of the well is in the range of from about 0.53 to 0.99. Various gases and liquids can be used to create the foam, but foams generally used in the art are made from nitrogen and water, in the presence of a suitable surfactant. The pressure at which the foam is pumped into the well is such that it will cause a fracture of the hydrocarbon-bearing formation. Additionally, the foam comes out of the well easily when the pressure is released from the wellhead, because the foam expands when the pressure is reduced.
Yet another fracturing technique has been that utilizing a liquified, normally gaseous fluid. U.S. Pat. No. 3,195,634 for example, discloses a method for treating a subterranean formation penetrated by a wellbore with a composition comprising a liquid--liquid mixture of carbon dioxide and water. The carbon dioxide is present in an amount equivalent to from about 300 to about 1500 SCF at 80.degree. F. and 14.7 psia per 42 gallons of water. The composition is injected into the formation under sufficient pressure to fracture the formation. The composition can include gelling agents and proppant materials. Upon pressure release at the wellhead, the liquid carbon dioxide vaporized and flows from the formation.
U.S. Pat. No. 3,310,112 discloses a method of fracturing a subterranean formation penetrated by a wellbore comprising introduction of a mixture of liquid carbon dioxide and a propping agent slurried in a suitable vehicle into the wellbore at a pressure sufficient to fracture the formation. The liquid carbon dioxide is present in an amount sufficient to provide at least five volumes of carbon dioxide per volume of slurried propping agent. After injection of the mixture of liquid carbon dioxide containing the propping agent slurried in a suitable vehicle, the pressure on the wellbore is released. The liquid carbon dioxide normally is heated sufficiently by the formation that upon pressure release the liquid changes to a gas. A substantial portion of the carbon dioxide then leaves the well and forces or carries out with it an appreciable amount of the oil or aqueous vehicle utilized to transport the proppant.
U.S. Pat. No. 3,368,627 discloses a method of treating a formation penetrated by a wellbore which consists essentially of injecting down the wellbore a fluid azeotropic mixture which has a critical temperature sufficiently high or a critical pressure sufficiently low to remain a liquid at the temperature and pressure existing during injection and treatment of the formation. The fluid mixture has critical properties such that a substantial portion of the injected fluid is converted to a gas upon a release of the pressure applied to the liquid during injection into the formation. The fluid mixture consists essentially of carbon dioxide and at least one C.sub.2 to C.sub.6 hydrocarbon.
U.S. Pat. No. 3,664,422 discloses a method of treating a subsurface earth formation penetrated by a wellbore comprising injection of a liquified gas together with a gelled alcohol into the formation at a pressure sufficient to fracture the formation. The liquified gas is returned from the formation by vaporization following pressure reduction on the wellbore. The gelled alcohol is removed by vaporization during subsequent production from the well leaving only the broken gelling agent in the formation.
U.S. Pat. No. 4,627,495 discloses a method of treating a subsurface formation and placing a proppant material utilizing a carbon dioxide or nitrogen containing fluid. The volume of carbon dioxide or nitrogen is adjusted as proppant is admixed with the fluid to maintain a substantially constant internal phase ratio in the treatment fluid.
A problem or concern that is not addressed by the foregoing is the potential for proppant to segregate within the fractures as a result of the different densities of the fluids and the varying proppant material concentrations contained in the fluids introduced into the subterranean formation. It would be desirable to provide a treatment which would substantially eliminate or avoid proppant material segregation while achieving appropriate proppant concentrations within the formation.